1. Field of the Invention
The present invention relates to an FF distortion compensation amplifier comprising a feedforward (hereinafter referred to as xe2x80x9cFFxe2x80x9d) loop that compensates for distortion occurring in a main amplifier, for example, intermodulation distortion, and in particular, to a control circuit and method that optimizes the FF loop.
2. Related Art of the Invention
Mobile communications base stations or the like transmit by radio multicarrier signals which have predetermined frequency intervals and which are each appropriately modulated, after radio frequency amplification. If an amplifier for use in radio-frequency amplification has insufficient linearity, various types of distortion for example, intermodulation distortion may occur, Such distortion hinders realization of normal and high-quality communications. Thus, for amplification of a multicarrier signal, the entire frequency band to which the multicarrier signal belongs must be appropriately linear, and a strict tolerance is specified for the distortion.
As an approach to implementation of a distortion compensation amplifier suitable for amplification of a multicarrier signal, an FF amplifying method is described in Japanese Patent Laid-Open No. 2000-196366 or the like.
With this FF amplifying method, if on a signal path from a signal input end through a main amplifier to a signal output end, i.e. a signal path over which signals to be amplified or amplified signals are transmitted (this signal path is hereinafter referred to as a xe2x80x9cmain linexe2x80x9d), a signal branched from a point located after the main amplifier and a signal branched from a point located before the main amplifier on the main line travel an equal electric length and have the same amplitude and opposite phases, then these signals can be coupled together to cause their carrier components to cancel each other to take out a signal corresponding to distortion caused by the main amplifier and its peripheral circuit.
The thus taken-out signal, which corresponds to the distortion, is passed through a distortion compensation loop and recoupled to the signal on the main line. When adjustment of amplitude or phase is appropriately carried out in the distortion compensation loop or main line so that a signal delay occurring on the main line is compensated for by a signal delay in the distortion compensation loop or that a distortion component contained in the signal on the main line and the signal obtained from the distortion compensation loop have the same amplitude and opposite phases, the above described signal recoupling operation compensates for the distortion occurring in the main amplifier.
FIG. 8 shows an example of a configuration of a conventional FF amplifier. In this figure, for example, a multicarrier signal input through a signal input end IN is branched into two by a hybrid HYB1. One of the branched signals is amplified by the main amplifier A1 and then reaches a hybrid HYB2. The other signal is supplied to the hybrid HYB2 via a delay line D1. The delay line D1 compensates for a signal delay that may occur in the main amplifier A1, and a signal delayed via the delay line D1 is coupled by the hybrid HYB2 to a signal containing distortion occurring in the main amplifier A1.
As described above, carrier components are mutually cancelled to take out (detect) distortion occurring in the main amplifier A1 by coupling a signal branched from an output signal from the main amplifier A1 to a signal obtained via the delay line D1 as described above. To achieve this, upon the coupling at the hybrid HYB2, the carrier components of the two signals must have opposite phases and the same amplitude and follow the same timing. The delay line D1 is means of allowing carrier components to follow the same timing, and a variable attenuator ATT1, a variable phase shifter PS1, and a control circuit 110 that adjusts and controls a signal attenuation G1 and a phase shift xcex81 in the variable attenuator ATT1 and the variable phase shifter PS1, respectively, to optimum values are means of allowing carrier components to have opposite phases and the same amplitude.
Next, in the FF amplifier shown in FIG. 8, carrier components amplified by the main amplifier A1 and containing distortion components are delivered to the hybrid HYB2. Then, in a distortion compensation loop L2, a signal containing no carrier signals but only distortion components is supplied to a hybrid HYB3 via the delay line D2. Simultaneously, the same signal is amplified by an auxiliary amplifier A2 and supplied to a hybrid. In the distortion compensation loop L2, the two signals have opposite phases and the same amplitude and follow the same timing upon coupling at HYB3 in order to compensate for (cancel) the distortion by coupling the signal from the delay line D2 and the signal from the auxiliary amplifier A2 together. The delay line D2 is means of allowing distortion components to follow the same timing, and the control circuit 110 that adjusts and controls a signal attenuation G2 in a variable attenuator ATT2 and a phase shift xcex82 in a variable phase shifter PS2 to optimum values is means of allowing distortion components to have opposite phases and the same amplitude.
In the FF amplifier shown in FIG. 8, an optimization process in the distortion compensation loop L2 is executed by inserting and detecting a pilot signal as described below. The control circuit 110 comprises a synchronous detector 138, an oscillator OSC2 that serves to generate a pilot signal, and an in-phase divider 128 that divides the signal from the oscillator OSC2 into two: a pilot signal and a reference signal REF. In the thus constructed distortion compensation loop L2, to cancel distortion by coupling a pilot signal from the delay line D2 and a pilot signal from the auxiliary amplifier A2 together, an output signal from the synchronous detector 138 adjusts and controls the amplitude attenuation G2 in the variable attenuator ATT2 and the phase shift xcex82 in the variable phase shifter PS2 to optimum values.
With the circuit constructed as described above, an FF amplifier can be actualized which is suitable for amplification of a multicarrier signal.
However, in the conventional example shown in FIG. 8, only one pilot signal is used, which has a frequency located a certain distance above or below the band in which the amplifier is operated. Accordingly, if the pilot signal has a frequency located above the operating band, the capability of removing or suppressing distortion in this frequency or frequencies located close thereto is optimized, whereas for a frequency band located below the band in which the amplifier is actually operated, the capability of removing or suppressing distortion is not always optimized. Thus, it has been desired to use both a pilot signal having a frequency located below the operating band and a pilot signal having a frequency located above the operating band.
Furthermore, according to an example of a conventional configuration such as the one shown in FIG. 8, an output signal from the distortion compensation loop L2 is supplied from a directional coupler DC4 to a band-pass filter BPF3 to extract only a pilot signal therefrom, which is then fed to the synchronous detector 38 as an error signal ERR. However, the output signal taken out from the directional coupler DC4 contains an amplified carrier signal in spite of the cancellation in hybrid HYB2. A filter with a very steep characteristic is required to remove this carrier signal component to extract a weak pilot signal. However, implementation of such a filter requires the physical size thereof to be increased, thereby making it difficult to miniaturize the circuit.
Further, to avoid this problem, it is contemplated that an output signal from the distortion compensation loop L2 may be down-converted so as to have a frequency in an IF band before filtering. However, this requires extra oscillators such as a local oscillator for down conversion and an IF local oscillator, thereby increasing the scale of the circuit.
In view of these problems, it is an object of the present invention to provide a feedforward distortion compensation amplifier or a control circuit or method for a feedforward distortion compensation amplifier which require a reduced number of oscillators and which enable optimization of distortion suppression over the entire frequency band of a multicarrier signal.
One aspect of the present invention is a method of controlling a feedforward distortion compensation amplifier comprising steps of
detecting a distortion component generated in a main amplifier by coupling a signal branched from a signal input to said main amplifier and containing a plurality of carriers of different frequencies to a signal branched from an output signal from said main amplifier so that the coupling causes the carrier components to cancel each other,
recoupling the signal resulting from said coupling to the output signal from said main amplifier,
and adjusting an amplitude and phase of at least one of the signals to be recoupled so that distortion components cancel each other upon recoupling,
wherein a first and second pilot signals obtained from a first and second pilot signal sources are inserted into the input signal to or output signal from the main amplifier, parts of the signal resulting from said recoupling are taken out through branching, and said signals taken out through branching are mixed with said first and second pilot signals, respectively, and thus down-converted to generate down convert signals so that control signals to adjust said amplitude and phase are generated according to said down convert signals.
Another aspect of the present invention is a circuit for controlling a feedforward distortion compensation amplifier comprising a distortion detection loop that detects a distortion component generated in a main amplifier by coupling a signal branched from a signal input to said main amplifier and containing a plurality of carriers of different frequencies to a signal branched from an output signal from said main amplifier so that the coupling causes the carrier components to cancel each other, a distortion compensation loop that recouples the signal resulting from said coupling to the output signal from said main amplifier, and means of adjusting an amplitude and phase of at least one of the signals to be recoupled so that distortion components cancel each other upon recoupling,
wherein a first and second pilot signals obtained from a first and second pilot signal sources are inserted into the input signal to or output signal from the main amplifier, parts of the signal resulting from said recoupling are taken out through branching, and said signals taken out through branching are mixed with said first and second pilot signals, respectively, and thus down-converted to generate down convert signals so that control signals to said means are generated according to said down convert signals.
Still another aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein said signal taken out through branching is passed through a predetermined frequency band to obtain a first pilot signal component that contains said first pilot signal but does not contain said second signal, and said first pilot signal component is mixed with said second pilot signal and thus down-converted to obtain a first down convert signal having a frequency amounting to a difference between a frequency of said first pilot signal and a frequency of said second pilot signal, and said signal taken out through branching is passed through another predetermined frequency band to obtain a second pilot signal component that-contains said second pilot signal but does not contain said first signal, and said second pilot signal component is mixed with said first pilot signal and thus down-converted to obtain a second down convert signal having a frequency amounting to said difference.
Yet still another aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein said signal taken out through branching is passed through said predetermined frequency band to obtain a first pilot signal component that contains said first pilot signal but does not contain said second signal, and said first pilot signal component is orthogonally mixed with said second pilot signal and thus down-converted to obtain a set of first down convert signals having a frequency amounting to a difference between a frequency of said first pilot signal and a frequency of said second pilot signal as well as quadrature phases, and said signal taken out through branching is passed through said another predetermined frequency band to obtain a second pilot signal component that contains said second pilot signal but does not contain said first signal, and said second pilot signal component is orthogonally mixed with said first pilot signal and thus down-converted to obtain a set of second down convert signals having a frequency amounting to said difference as well as quadrature phases.
Still yet another aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein said first down convert signal and/or said second down convert signal is synchronously detected to generate said control signal using a signal for synchronized detection of the difference between the frequency of said first pilot signal and the frequency of said second pilot signal, the synchronized detection signal supplied by synchronized detection signal generating means of generating the synchronized detection signal using said first pilot signal and said second pilot signal.
A further aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein said set of first down convert signals having quadrature phases and/or said set of second down convert signals having quadrature phases are synchronously detected to generate said control signal using a signal for synchronized detection of the frequency difference between the frequency of said first pilot signal and the frequency of said second pilot signal, the synchronized detection signal supplied by synchronized detection signal generating means of generating the synchronized detection signal using said first pilot signal and said second pilot signal.
A still further aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein a signal obtained by synthesizing in-phase components of said set of first down convert signals having quadrature phases and of said set of second down convert signals having quadrature phases, and a signal obtained by synthesizing quadrature components of said set of first down convert signals having quadrature phases and of said set of second down convert signals having quadrature phases are each synchronously detected to generate said control signal using a signal for synchronized detection of the frequency difference between the frequency of said first pilot signal and the frequency of said second pilot signal, the synchronized detection signal is supplied by synchronized detection signal generating means of generating the synchronized detection signal using said first pilot, signal and said second pilot signal.
A yet further aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein said set of first down convert signals having quadrature phases and/or said set of second down convert signals having quadrature phases are respectively level detected to generate said control signal.
A still yet further aspect of the present invention is a circuit for controlling a feedforward distortion compensation amplifier, wherein said synchronized detection signal generating means mixes said first pilot signal and said second pilot signal together, allows passage of a part of said mixed signal which has a frequency amounting the difference between the frequency of said first pilot signal and the frequency of said second pilot signal, and distributes the signal obtained to a plurality of paths.
An additional aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein said first down covert signal and said second down convert signal are alternately switched to be detected.
A still additional aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein time for detection using said first down convert signal and time for detection using said second down convert signal are determined by weighting.
A yet additional aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein the frequencies of said first and second pilot signals are determined on the basis of all of said carriers, and said weighting is determined on the basis of those of all of said carriers which are actually used.
A still yet additional aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein elements constituting oscillators for said first and second pilot signal source, elements constituting a band-pass filter that takes out said first pilot signal by passing the signal therethrough, and elements constituting a band-pass filter that takes out said second pilot signal by passing the signal therethrough have substantially the same temperature characteristic.
A supplementary aspect of the present invention is the circuit for controlling a feedforward distortion compensation amplifier, wherein elements constituting oscillators for said first and second pilot signal source, elements constituting a band-pass filter that takes out said first pilot signal by passing the signal therethrough, and elements constituting a band-pass filter that takes out said second pilot signal by passing the signal therethrough are installed on ovens subjected to the same temperature control.
A still supplementary aspect of the present invention is a feedforward distortion compensation amplifier, comprising:
a distortion detection loop that detects a distortion component generated in a main amplifier by coupling a signal branched from a signal input to said main amplifier and containing a plurality of carriers of different frequencies to a signal branched from an output signal from said main amplifier so that the coupling causes the carrier components to cancel each other;
a distortion compensation loop that recouples the signal resulting from said coupling to the output signal from said main amplifier; and
means of adjusting an amplitude and phase of at least one of the signals to be recoupled so that distortion components cancel each other upon recoupling,
wherein the amplifier comprises the circuit controlling a feedforward distortion compensation amplifier.